Various bipyridinium salts and polymers which undergo reversible oxidation-reduction (redox) changes in spectral absorption characteristics are known and described in the art. Specifically, these compounds have been used as mediators in electro-chemical reduction and oxidation of biological molecules.
The 4,4'-bipyridinium unit is commonly known as a "viologen" and it undergoes two separate one-electron reduction steps from the dication to the cation to the neutral (dihydrobipyridyl) species as shown, for example, in U.S. Pat. No. 3,856,714, which is incorporated herein by reference. The dicationic and neutral forms are colorless, but the cationic form is a deep blue-violet color. This property has resulted in the use of such viologens as photochemical or redox indicators and as the visual element in alphanumeric image displays.
Polymers containing repeating viologen units have been previously described in the art but have been produced by alkylation or interfacial polycondensation. See U.S. Pat. Nos. 3,641,034; 3,671,250; 3,694,384; and 3,856,714.
Several prior patents describe the electrolytic polymerization of monomers of viologens or similar compounds. For example, U.S. Pat. No. 3,574,072 describes generally the polymerization of numerous heterocyclic compounds.
U.S. Pat. No. 3,854,794 discloses the use of viologens in image display cells wherein a film of the cation radical salt (not a polymer) is produced on a cathode at rather high potentials. The film deposition process is described as being reversible by reversing the electrode polarity. A literature article in the Journal of the Electrochemical Society (Vol. 124, No. 5, May 1977, p. 637) discloses additional insoluble, heptyl viologen compounds for image displays.
In the oxidation and reduction of biological molecules it is usually found that solid metal electrodes are not capable of directly transferring electrons at a high rate. Viologens may therefore be used as mediators to intermediate the transfer of electrons from the electrode to the molecule. Unfortunately, the mediators tend to hamper optical studies of the redox reactions due to their large absorbances and electron paramagnetic resonance signals.